1. Field of the Invention
The present invention relates to a new process for preparing a polyamide by direct polycondensation reaction of a diamine component and a dicarboxylic acid component. More particularly, the present invention relates to a process for preparing a polyamide economically which comprises mixing a molten dicarboxylic acid component containing adipic acid as a main component and a diamine component containing m-xylylene diamine as an another main component, and reacting them directly at a controlled temperature and at atmospheric pressure.
2. Description of the Prior Art
In general, a polyamide is produced by subjecting an aqueous solution of the salt formed from a dicarboxylic acid and a diamine (the so-called nylon salt) to polycondensation reaction at a temperature just sufficient to polycondense the salt under pressure while distilling water away, which is used as a solvent of the nylon salt, and is formed by polycondensation of the salt.
The above conventional method requires not only much heat and extremely long periods of time for the reaction to remove water, but also the yield obtained per one batch is low, and thus it is uneconomical. Furthermore, in order to produce a polyamide having high quality by this conventional method, there are many problems to be solved. For example, polymer deposits are formed on the wall of the reaction vessel owing to the remarkable change in the liquid level of the reaction solution during the reaction, and the deposited polymer can be locally overheated and tends to degrade.
As a method of eliminating those drawbacks, it has been proposed that the nylon salt be directly subjected to polycondensation reaction without using any solvent. See, Japanese Patent Publication (Kokoku) Nos. 35-15700 and 43-22874. However, these methods are not much more efficient, because they require the steps for the isolation of the nylon salt and the purification thereof in using the same. Also U.S. Pat. No. 2,840,547 discloses a method in which a diamine and a dicarboxylic acid are directly mixed and the mixture is brought to polycondensation reaction under pressure. Further, Japanese Patent Publication (Kokai) No. 48-12390 discloses a method in which a molten diamine containing a small amount of water is mixed with a molten dicarboxylic acid at a temperature of less than 220.degree. C. at atmospheric pressure, while the polycondensation reaction is effected under such conditions that the polycondensation reaction proceeds as slowly as possible.
However, a method comprising subjecting only a diamine and a dicarboxylic acid directly to polycondensation reaction at atmospheric pressure to produce a polyamide economically has, surprisingly, not been put into practice until now. It is considered to be mainly due to the following reasons.
Firstly, in the case that the mixture of a dicarboxylic acid and a diamine is directly subjected to polycondensation reaction at atmospheric pressure, if the reaction mixture containing the starting materials is maintained in a uniformly fluidized state, it becomes difficult to avoid the loss of the diamine by evaporation. This causes the molecular weight of the product to vary from batch to batch. In order to prevent this loss, the reaction system must be kept under pressure with steam. This means naturally that the apparatus for the polycondensation reaction must withstand the applied pressure, and the procedures of the polycondensation reaction must include both steps of keeping the reaction system under pressure and under reduced pressure. This is disadvantageous because the apparatus and operation are complicated and, also, the reaction time is long, in comparison with the case of carrying out direct polycondensation reaction at atmospheric pressure.
Secondly, if a diamine is directly polycondensed with a dicarboxylic acid, the viscosity of the polyamide produced markedly increases as the reaction proceeds. The increase in viscosity causes the overall heat transfer coefficient, U, of the reaction vessel to fall so that the time required for increasing or decreasing the temperature of the vessel contents becomes relatively long. So, economical production of polyamide is markedly impeded by restrictions on size imposed on the apparatus used for such reaction.
As described above, in the industrial production of polyamide by a direct polycondensation process, improvements have been desired in practical use.